One popular form of sterilizable container often used to store medical or surgical instruments is a pouch comprised of two sheets of plastic, for example low density polyethylene, positioned face to face and sealed around their common periphery with heat seals. An access opening is cut in one of the sheets, and the opening covered with a porous membrane sealed to the pouch around the opening. The porous membrane in this construction serves two functions; it covers the access opening, and it provides a means to sterilize the contents of the pouch. The membrane is sealed to the pouch with peelable seals so that it can be easily peeled from the pouch, and the contents removed through the access opening. Also, the membrane is made porous in order that the contents of the pouch can be sterilized using a sterilizing gas (chemical or steam) after the pouch has been sealed. The porosity of the membrane is such that the sterilizing gas can pass through it, yet the membrane forms a sterile barrier against bacteria or the like. Porous membranes for such pouches (called "tear strips") are commonly made of surgical grade paper or spunbonded olefin, which is sold by the Du Pont Company under the trademark Tyvek.
The "breathability" of tear strips is an important issue in the construction of sterilizable pouches. Is is desirable that the tear strip offer as little resistance as possible to the flow of the sterilizing gas in order that the sterilization cycle be as short as possible. Unfortunately, in order to form a peelable seal with low density polyethylene, paper or Tyvek must be coated or otherwise treated, and the required treatment is such that the membrane's porosity is substantially reduced. The sterilization cycle using such tear strip materials is usually substantially longer than desired. Consequently, it is preferred that uncoated tear strips be used.
This problem was addressed by the present inventor in U.S. Pat. No. 4,367,816. The solution there presented was to use a two layered tear strip, with the top layer being an uncoated porous membrane, and the second being an interlayer between the porous membrane and the pouch proper. The interlayer is a sheet (generally plastic) which can form a peelable seal to the material of the pouch, and also seal to the uncoated membrane. The interlayer disclosed in the '816 patent was provided with small holes, perforations, or a slit to allow the sterilizing gas to pass.
While the prior invention was generally a great improvement over the prior art, a problem can possibly arise in using the construction disclosed therein, particularly with larger pouches. When the sterilzing gas is flowing outward, the interlayer sheet tends to press against the porous membrane, and in some cases this could result in reducing the area of the membrane through which the gas is flowing, and thereby impede gas flow.
Using existing manufacturing techniques, it has not been practical to cut large openings in the interlayer. If the open area of the interlayer is made large, i.e., if the width of the opening is made more than about 25% of the width of the sheet, certain manufacturing difficulties arise, as will be discussed further below. These difficulties are overcome by the method of manufacturing disclosed in this specification, and using the method disclosed, it becomes practical to fabricate pouches wherein the breathing area for outgoing gases is substantially the same as for incoming gases. The terms "length" and "width" used in this document refer to the direction that the webs in the pouch making machine move during fabrication, and crosswise to the machine direction, respectively. Using this convention, the resulting pouches may very well (in fact are likely to) have "widths" which are greater than their "lengths".